The present invention relates to improvements in motion-detection systems of the PIR (passive infrared radiation) variety. More particularly, it relates to apparatus for supervising a PIR motion-detection system to alert the system user of a change in sensitivity of the IR-sensing component.
It is well known in the art to detect the presence of an intruder (or pedestrian) in a region under surveillance by detecting a change in ambient temperature caused by the intruder's own body heat. Infrared detection systems of this type typically comprise a pyroelectric sensor, spectrally tuned to a wavelength of about 10 microns, and an optical system for focusing radiation from different fields of view onto the sensor. The sensor itself commonly comprises two or more IR-sensitive elements which are spaced apart and, together with the optical system, define a plurality of narrow fields of view. As the intruder passes through these fields of view, each element produces a signal which, ideally, is suitably processed to detect only the object of interest, thereby avoiding false alarms.
PIR systems of the above type are "passive" in nature in that they rely only on the IR energy produced by the object of interest to produce an alarm. Unlike microwave, ultrasonic and photoelectric detection systems which actively transmit energy into a region of interest and look for intruder-produced changes in the frequency and/or level of such energy, one cannot readily monitor or "supervise" the operability of a PIR system by passively monitoring the output of such systems. In PIR detection systems, there is no output until a target (i.e., an IR transmitter) enters the system's field of view. Thus, to assure that a PIR system is indeed functional, one must actively simulate a target of interest and determine whether an alarm signal is produced in response to that target. It is known, for example, to simulate a target by periodically irradiating a PIR sensor with a heating element, a light-emitting diode (LED) or a light bulb. See, for example, the disclosures of my commonly assigned U.S. Pat. No. 5,093,656, as well as the disclosure of U.S. Pat. No. 3,928,849, issued to F. Schwarz.
The "active" supervisory schemes described in the above patent references are designed to detect only a catastrophic failure of the system, as may be occasioned by a broken wire or a non-functioning system component. They are neither designed nor intended to detect a change (either increase or decrease) in the system sensitivity so as to signal a change in detection range of the system. Not having this capability can be problematic. For example, an unexpected increase in system sensitivity, as may occur with a change in amplifier gain, will give rise to an increased detection range and attendant false alarms. On the other hand, a decrease in sensitivity, as may result from a degrading sensor or electrical components, results in a reduced detection range and a corresponding loss of protection. Often, it is desirable to produce a "trouble" signal when the system sensitivity has changed by a predetermined amount vis-a-vis a nominal level.